Bottom-hinged, clamshell-type aircraft fuselage doors that integrally include steps to allow passengers to board and disembark when the door is opened are colloquially known as “airstair” doors. Aircraft provided with airstair doors can thus provide service to many less populated airport environments since a fixed-based gantry platform to allow passengers to board and disembark is not necessarily required. For these reasons, many regional transport and general aviation aircraft are equipped with airstair fuselage doors.
Since aircraft fuselage doors are bottom hinged, some form of weight counterbalance mechanism is typically required to assist an operator (typically on-board personnel) to open and close the door. Various mechanisms based on hydraulic, electric or spring actuation concepts are therefore known and used in aircraft designs. In the case of hydraulic actuation, for example, systems are known which possess substantial load capacities and relatively simplistic operational modes, including automated push-button door opening and closing. Similar automated operations based on electric motor actuation systems are also known whereby an electrical actuator or motor performs the door movement. However, there are actuation cycle limitations imposed on both hydraulic and electric actuation concepts due to the necessary recharge of hydraulic accumulators and/or on-board batteries that are required for proper operation. In addition, there are substantial space penalties associated with the incorporation of hydraulic and electric door actuation mechanisms that may preclude their being used on certain types of aircraft designs.
U.S. Pat. No. 5,704,569 to Daniels (the entire content of which is expressly incorporated hereinto by reference) describes a mechanical counterbalance mechanism for upwardly and inwardly operated aircraft cargo doors. The counterbalance mechanism as disclosed therein includes a guide tube having a rod that extends outwardly therefrom and compression springs which oppose the outward movement of the rod. A linkage system includes a bellcrank and a push rod which connects the guide tube to the cargo door to assert a counterbalancing force during door opening and closing.
While the counterbalance mechanism as described in the Daniels '569 patent is suitable for upwardly and inwardly operated cargo doors, it is not conveniently adapted for use with cargo airstair doors. Therefore, there exists continued need for a counterbalancing mechanism that may be employed for bottom-hinged clamshell-type aircraft airstair doors. It is therefore towards providing solutions to such a need that the embodiments of the present invention are directed.